The field of the invention is inhibition of accelerated atherosclerosis.
Accelerated atherosclerosis is a proliferative process leading to vascular stenosis which commonly occurs following percutaneous transluminal coronary angioplasty (PTCA), heart transplantation, and coronary vein graft. The process is characterized by platelet activation, thrombus formation, and smooth muscle cell hyperproliferation. Accelerated atherosclerosis leads to vascular stenosis in 35-50% of the patients who have undergone PTCA.
Injury of the vascular endothelium is thought to be the initiating event in both spontaneous and accelerated atherosclerosis. Accelerated atherosclerosis commonly occurs following denuding endothelial injury; such injury can be caused by transluminal coronary angioplasty, heart transplantation, and coronary vein graft and is sometimes accompanied by damage to the intima and media. In contrast, spontaneous atherosclerosis is thought to be caused by functional impairment of the endothelium caused by non-denuding, chronic damage of the vascular wall. In both cases, smooth muscle cell hyperproliferation is one of the primary causes of vascular stenosis.
In accelerated atherosclerosis, endothelial damage is followed by accumulation of platelets, monocytes, and lymphocytes; thrombosis; smooth muscle cell migration and proliferation; and lipid accumulation. Interactions between blood borne cells and injured endothelial tissue are thought to create an environment conducive to smooth muscle cell proliferation. Important contributors to this process include, the loss of the inhibitory effect of an intact endothelial layer; release of mitogenic factors by platlets monocytes/macrophages, injured endothelial cells, and smooth muscle cells; activation of T-lymphocytes and monocytes/macrophages; and thrombosis.
Each of the cell types involved in accelerated atherosclerosis may play a role in smooth muscle cell proliferation. Platelets produce platelet-derived growth factor (PDGF), a potent smooth muscle cell mitogen; damaged endothelium, monocytes/macrophages, and smooth muscle cells can all generate a PDGF-like growth factor. Activated monocytes/macrophages, stimulated endothelial cells, and stimulated smooth muscle cells produce interleukin-1, a factor which stimulates proliferation of endothelial cells and smooth muscle cells. Other factors produced by platelets and atheroma cells which are mitogenic for myocytes include: .alpha.-fibroblast growth factor, .beta.-fibroblast growth factor, serotonin, and thrombospondin.
Activated T-lymphocytes are present in atherosclerotic plagues and may play a role in several of the processes involved in accelerated atherosclerosis. They release lymphokines and interleukins which can regulate lipoprotein uptake by monocytes/macrophages and which may up-regulate expression of growth factor receptors on smooth muscle cells. Activated T-lymphocytes also release chemotactic factors which enhance migration and adhesion of monocytes/macrophages.
Methods for prevention of accelerated atherosclerosis are generally designed to decrease either thrombogenesis or cell proliferation. Using a baboon vascular graft model, Hanson et al. (J. Clin. Invest. 81:149, 1988) demonstrated that anti-glycoprotein IIb/IIIa monoclonal antibodies reduced acute graft closure secondary to thrombosis by 72%. Using the same monoclonal antibody, Bates et al. (Circulation 78:II-289, 1988) demonstrated a reduction in coronary thrombosis after coronary angioplasty in dogs. Badimon et al. (J. Am. Coll. Cardiol. II Supl. A:30A, 1988) demonstrated an 81% reduction in platelet deposition on de-endothelialized vessel wall in swine treated with a monoclonal antibody directed against von Willebrand factor. Heras et al. (Circulation 79:657, 1981) found that recombinant hirudin significantly decreases platelet and fibrinogen deposition in swine subjected to balloon angioplasty. Sarenbock et al. reported similar results following air desiccation injury in rabbits (Circulation 82:III-208, 1990). Thrombin inhibitors have been shown to reduce thrombosis following carotid artery endarterectomy in baboons (Schneider et al., Circulation 78:II-311, 1988; Jang et al., Circulation 78:II-311, 1988). Acetylsalicylic acid pre-treatment has been shown to reduce platelet accumulation in patients who have undergone coronary angioplasty (Cunningham et al., Radiology 151:487, 1984). A placebo controlled study in 376 patients demonstrated that while an aspirin-dipyridamide "antiplatelet regimen before and after PTCA did not reduce the six-month rate of restenosis after successful coronary angioplasty, it markedly reduced the incidence of transmural myocardial infarction during or soon after PTCA" (Schwartz et al., N. Engl. J. Med. 318:1714, 1988).
Heparin is commonly used following coronary angioplasty to reduce the incidence of acute thrombotic occlusion. Heparin may also have antiproliferative activity, and thus may be useful in prevention of restenosis. Heparin has been shown to reduce platelet accumulation on denuded neointima (Mustard, Ann. R. Coll. Physicians Surg. Can. 14:22, 1981). A study found that intravenous heparin in doses large enough to cause continuous anticoagulation reduced myointimal thickening in rats whose carotid arteries had been injured (Clowes et al., Nature 265:625, 1977). An in vivo study found that heparin inhibits smooth muscle cell proliferation which occurs after denudation of endothelium by air-drying the rat carotid artery; this effect does not depend on anticoagulant activity (Guyton et al., Circ. Res. 46:625, 1980). In vitro studies of cultured rat smooth muscle cells demonstrated that heparin, in either its high anticoagulant form or its non-anticoagulant form, significantly inhibits cell proliferation (Hoover et al., Circ. Res. 47:578, 1980). Gordon et al. (Circulation 76:IV-213, 1987) demonstrated that arterial smooth muscle cell proliferation following balloon catheter injury in rats was significantly reduced by administration of low molecular weight heparin.
Methotrexate and azathioprine have been investigated as antiproliferative agents for treatment of restenosis (Murphy et al. Circulation 82:III-429, 1990; Muller et al. Circulation 82:III-429, 1990).
Wai et al. (Circulation 82:III-208, 1990) found that a hybrid protein consisting of the ribosome inhibitor, saponin, fused to basic fibroblast growth factor (FGF) killed proliferating, FGF receptor-expressing smooth muscle cells, but not quiescent receptor negative cells; this same hybrid protein inhibited intimal thickening following vascular injury.